The invention relates to a power steering system provided with a vane pump.
Generally, a power steering system is arranged such that oil discharged from a vane pump is supplied to or displaced from a power cylinder through a power steering servo valve, thereby imparting an assistance to a steering operation. The vane pump comprises a cam ring having a substantially elliptical cam surface, a rotor which is adapted to rotate inside the cam ring and a plurality of vanes adapted to move back and forth within radial slits formed in the rotor. A pumping function is provided by increasing and decreasing the volume of a pump chamber which is defined between a pair of adjacent vanes as the rotor rotates. During the rotation of the rotor, discharged oil is introduced to act upon the rear surface of the vanes so as to maintain the individual vanes positively in sliding contact with the cam surface of the cam ring.
In a conventional power steering system as described above, the discharge per revolution of the vane pump remains constant, and accordingly it is required to discharge a flow rate during a high speed operation, all of which results in an increased loss of driving horsepower. The introduction of the discharged pressure oil to act upon the rear surface of the vane causes the force, which urges the vane against the cam surface in a suction region to be increased, resulting in an abrasion caused by the relative sliding movement between the cam surface and the tip of the vanes. This made it impossible to chamfer the cam surface or to provide a suction opening in order to achieve a higher suction efficiency. The increased force with which the vane is urged against the cam surface increases the resistance presented to the sliding movement, thus causing a disadvantage that the driving horsepower is wasted.